Ever since Kirchhoff and Bunsen (1860) introduced spectral
analysis, that extremely important aid to investigation has
produced the finest results. To begin with, material was
collected and spectra were investigated not only from terrestrial
objects but also from the heavenly bodies. There was a splendid
harvest. Then came the second stage of research. Attempts were
made to find regularities in the structure of the spectra. To
begin with, it was natural to try to compare the different
spectral lines which are emitted by a glowing gas with the
different notes that could be produced by a vibrating solid. The
vibrating bodies in a glowing gas would in that case be its atoms
and molecules. But little progress could be made on this track.
It was necessary to fall back on another method, namely to try by
calculation to establish the connection between the various
vibrations which could be emitted by a gas. Hydrogen ought to be
the simplest of all gases. The Swiss Balmer in 1885 found a
simple formula for the connection mentioned between the lines of
hydrogen as then known. There followed a large number of
investigators, such as Kayser and Runge, Ritz, Deslandres, and
especially our compatriot Rydberg, who sought for similar
regularities in the spectra of the other chemical elements.
Rydberg succeeded in representing their light vibrations by means
of formulae which exhibited a certain resemblance to Balmer's
formula. These formulae contain a constant which has afterwards
acquired extremely great importance and has been recorded amongst
the universal and fundamental values of physics under the name of
the Rydberg constant.

Now, if it were possible to obtain an idea of the structure of
the atom, of course, that would form a good starting-point to
create a conception of the possible light vibrations that can be
emitted by an atom of hydrogen. Rutherford, who has to such an
extraordinary degree wrung their secrets from the atoms, had
constructed such "atom models". According to his conception, the
atom of hydrogen should consist of a positive nucleus, with a
unit charge, of extremely small dimensions, and about this a
negatively charged electron should describe an orbit. As probably
only electric forces are at work between the nucleus and the
electron, and as these electric forces follow the same law as the
attraction of gravity between two masses, the path of the
electron ought to be elliptical or circular, and the nucleus to
be situated either in one of the foci of the ellipse or in the
centre of the circle. The nucleus would be comparable to the sun
and the electron to a planet. In accordance with the classical
theory of Maxwell, therefore, these orbit movements should emit
rays and consequently cause a loss of energy, and the electron
would describe smaller and smaller tracks with a declining period
of revolution and finally rush in towards the positive nucleus.
Thus the track would be a spiral, and the rays of light emitted,
which will require a steadily declining period of vibration,
would correspond to a continuous spectrum, which, of course, is
characteristic of a glowing solid or liquid body, but not at all
of a glowing gas. Consequently, either the atom model must be
false, or else the classical theory of Maxwell must be incorrect
in this case. Ten years or so previously there would have been no
hesitation in the choice between these alternatives, but the atom
model would have been declared to be inapplicable. But in 1913,
when Bohr began to work at this problem, the great physicist
Planck of Berlin had traced his
law of radiation, which could be explained only on the
assumption, which was in conflict with all preceding notions,
that the energy of heat is given offin the form of "quanta", that
is to say small portions of heat, just as matter consists of
small portions, i.e. the atoms. With the help of this assumption
Planck succeeded, in complete accordance with experience, in
calculating the distribution of energy in radiation from a
hypothetically completely black body. Afterwards (in 1905 and
1907) Einstein had perfected the
quantum theory and deduced therefrom several laws, such as the
diminution of the specific heat of solid bodies with declining
temperature and the photoelectric effect, for which discovery he
has this day been awarded the Nobel Prize.

Accordingly, Bohr had no need to hesitate in his choice: he
assumed that Maxwell's theory does not hold good in the present
case, but that the atom model of Rutherford is correct. Thus the
electrons do not emit light when they move in their tracks round
the positive nucleus, tracks which we begin by assuming to be
circular. The emission of light would take place when the
electron jumps from one track to another. The quantity of energy
which is thus radiated is a quantum. As, according to Planck, the
quantum of energy is the product of the number of light
vibrations with the Planckian constant, which is denoted by the
letter h, it is possible to calculate the number of
vibrations which corresponds to a given passing from one orbit to
another. The regularity which Balmer found for the spectrum of
hydrogen requires that the radii of the different orbits should
be proportional to the squares of the whole numbers, that is to
say as 1 to 4 to 9, and so on. And indeed Bohr succeeded, in his
first treatise on this question, in calculating the Rydberg
constant from other known magnitudes, namely the weight of an
atom of hydrogen, the Planckian constant, and the value of the
electric unit of charge. The difference between the value found
by observation and the calculated value of the Rydberg constant
amounted to only 1 percent; and this has been diminished by more
recent measurements.

This circumstance at once attracted the admiring attention of the
scientific world to Bohr's work and made it possible to foresee
that he would to a great extent solve the problem before him.
Sommerfeld showed that what is known as the fine structure of the
hydrogen lines, by which is meant that the lines observed with a
strongly dispergent spectroscope are divided up into several
closely adjacent lines, can be explained in accordance with
Bohr's theory in the following way. The various stationary tracks
for the movement of the electrons - if we leave out of account
the innermost one, which is the ordinary one, and is called the
"orbit of rest" - may be not only circular but also elliptical,
with a major axis equal to the diameter of the corresponding
circular orbit. When an electron passes from an elliptical orbit
to another track, the change in the energy, and consequently the
number of vibrations for the corresponding spectral lines, is
somewhat different from what it is when it passes from the
corresponding circular orbit to the other track. Consequently we
get two different spectral lines, which nevertheless lie very
close to one another. Yet we observe only a smaller number of
lines than we should expect according to this view of
things.

The difficulties thus revealed, however, Bohr succeeded in
removing by the introduction of what is known as the principle of
correspondence, which opened up entirely new prospects of great
importance. This principle to some extent brings the new theory
nearer to the old classical theory. According to this principle,
a certain number of transitions are impossible. The principle in
question is of great importance in the determination of the
tracks of electrons which are possible within atoms that are
heavier than the atom of hydrogen. The nuclear charge of the atom
of helium is twice as great as that of the atom of hydrogen: in a
neutral condition it is encircled by two electrons. It is the
lightest atom next that of hydrogen. It occurs in two different
modifications: one is called parhelium, and is the more stable,
and the other is called orthohelium - these were supposed at
first to be two different substances. The principle of
correspondence states that the two electrons in parhelium in
their tracks of rest run along two circles, which form an angle
of 60° to one another. In orthohelium, on the other hand,
the tracks of the two electrons lie in the same plane, the one
being circular, while the other is elliptical. The following
element with an atomic weight which is next in magnitude to
helium is lithium, with three electrons in a neutral state.
According to the principle of correspondence, the tracks of the
two innermost electrons lie in the same way as the tracks of the
two electrons in parhelium, while the track of the third is
elliptical and is of far greater dimensions than the inner
tracks.

In a similar manner Bohr is able, with the help of the principle
of correspondence, to establish, in the most important points,
the situation of the various tracks of electrons in other atoms.
It is on the positions of the outermost electron tracks that the
chemical properties of the atoms depend, and it is on this ground
that their chemical valency has partly been determined. We may
entertain the best hopes of the future development of this great
work.

Professor Bohr. You have carried to a
successful solution the problems that have presented themselves
to investigators of spectra. In doing so you have been compelled
to make use of theoretical ideas which substantially diverge from
those which are based on the classical doctrines of Maxwell. Your
great success has shown that you have found the right roads to
fundamental truths, and in so doing you have laid down principles
which have led to the most splendid advances, and promise
abundant fruit for the work of the future. May it be vouchsafed
to you to cultivate for yet a long time to come, to the advantage
of research, the wide field of work that you have opened up to
Science.